1. Field of the Invention
The present invention relates to semiconductive ceramics having negative resistance temperature characteristics, and more particularly, the invention relates to a semiconductive ceramic containing a lanthanum-cobalt-based oxide as a major constituent.
2. Description of the Related Art
Conventionally, elements composed of semiconductive ceramics having negative resistance temperature characteristics, in which the resistance decreases as the temperature increases (hereinafter referred to as xe2x80x9cNTC elementsxe2x80x9d), have been used for preventing initial overcurrents. Since the NTC elements have high resistance at room temperature, overcurrents at startup are prevented. As the temperature increases due to self-heating, the resistance decreases, and in the steady state, electric power consumption decreases. Such NTC elements are used for preventing rush currents, delaying the start of motors, etc.
For example, an overcurrent in a switching device may flow at the moment when the switch is turned on. As an element for absorbing such an initial rush current, an NTC element for preventing rush currents is used.
In a gear device in which a lubricant is fed after a motor is started, it is preferable that the rotational speed of the gear be gradually increased by the drive motor so that the lubricant can be spread over the entire gear device. As an element for delaying the start of the motor for a predetermined period of time, an NTC element for delaying the start of motors is used.
As materials having negative resistance temperature characteristics, rare-earth transition element-based oxides are disclosed in xe2x80x9cPhys. Rev. B6 [3]1021xe2x80x9d by V. G. Bhide, D. S. Rajoria, et al. published in 1972.
Furthermore, Japanese Unexamined Patent Publication No. 7-176406 discloses LaCo-based oxides to which Si, Zr, Hf, Ta, Sn, Sb, W, Mo, Te, Ce and the like are added as semiconductive ceramics in which the resistance is decreased and the B constant is increased.
However, the conventional semiconductive ceramics have the following drawbacks. Because of insufficient reaction between a lanthanum oxide or lanthanum compound and a cobalt oxide, the unreacted lanthanum oxide may remain in a semiconductive ceramic even after firing is performed, resulting in a deviation in molar ratio. The lanthanum oxide in semiconductive ceramics swells over time, which is disadvantageous in view of stability of various properties of ceramic sintered compacts.
It is an object of the present invention to provide a semiconductive ceramic in which deviation in molar ratio between amounts of lanthanum and cobalt is minimized and properties are more stabilized.
In a first aspect of the present invention, a semiconductive ceramic is produced by sintering a semiconductive ceramic material containing a mixed powder of a lanthanum oxide or a lanthanum compound and a cobalt oxide. The mixed powder has a specific surface area of about 3 m2/g or more, an average primary particle size of about 2.0 xcexcm or less, and an average secondary particle size of about 10 xcexcm or less.
In a second aspect of the present invention, a semiconductive ceramic is produced by sintering a semiconductive ceramic material containing a mixture of a powder of a lanthanum oxide or a lanthanum compound and a powder of a cobalt oxide. The powder of the lanthanum oxide or the lanthanum compound has a specific surface area of about 1 m2/g or more, an average primary particle size of about 2.0 xcexcm or less, and an average secondary particle size of about 10 xcexcm or less. The powder of the cobalt oxide has a specific surface area of about 5 m2/g or more, an average primary particle size of about 2.0 xcexcm or less, and an average secondary particle size of about 10 xcexcm or less.
As used herein, the average particle size is size measured by laser diffraction scattering of a dispersion of the material being determined (i.e., either the mixture or individual constituents of the mixture). Thus, about 100-1000 mg of the powder material to be measured is dispersed in about 250-300 ml of water containing 0.5% of sodium hexametaphosphate and then ultrasound (100W for 3 minutes) is applied before measuring. To determine primary size, the material is jet milled to break up agglomerates before being dispersed whereas to determine secondary size, it is not jet milled.
By specifying the compositions as described above, deviation in molar ratio between amounts of lanthanum and cobalt can be minimized. That is, by increasing the specific surface area of the semiconductive ceramic material and decreasing the particle size, reactivity between lanthanum and cobalt can be increased and the amount of unreacted lanthanum can be decreased during sintering.
In a third aspect of the present invention, a semiconductive ceramic element includes a semiconductive ceramic according to the first aspect or the second aspect of the present invention and electrodes formed at surfaces of the semiconductive ceramic. By constructing such a structure, the semiconductive ceramic element has a high B constant and low resistance in the steady state.
The semiconductive ceramic element is preferably used for preventing rush currents or delaying the start of motors. In such applications, electrical properties of the semiconductive ceramic element can be utilized effectively.